Surface treatment of tinplate for improving sulfur resistance

ABSTRACT

Surface treatment of tin-plated steel for food can and the like for improving sulfur resistance, by applying chromate treatment on the plated tin, immediately after electroplating thereof, in an aqueous solution containing 6-valent chromium ions, and then heating at a temperature above 100*C.

SURFACE TREATMENT OF TINPLATE FOR IMPROVING SULFUR RESISTANCE Inventors: Shunichi Harada; Takahisa Yoshihara, Chiba, both of Japan Assignee: Kawasaki Steel Corporation, Kobe,

Japan Filed: July 9, 1973 Appl. No.: 377,397

US. Cl 148/62; 204/37 T; 204/38 A Int. Cl. C23F 7/26 Field of Search 204/37 T, 35 R; 148/62 References Cited UNITED STATES PATENTS 4/1950 Prust 204/37 T 111' 3,890,164 June 17, 1975 2,820,747 1/1958 Ffedrickson 204/37 T Primary Examiner-Ralph S. Kendall Assistant Examiner Charles R. Wolfe, .I r.

ABSTRACT Surface treatment of tin-plated steel for food can and the like for improving sulfur resistance, by applying chromate treatment on the plated tin, immediately I after electroplating thereof, in an aqueous solution containing 6-valent chromium ions, and then heating at a temperature above 100C.

5 Claims, No Drawings SURFACE TREATMENT OF TINPLATE FOR IMPROVING SULFUR RESISTANCE BACKGROUND OF THE INVENTION 1. Field of the Invention:

This invention relates to surface treatment of tinplate for improving sulfur resistance, and more particularly to surface treatment of tin-plated steel for food can and the like for improving sulfur resistance thereof.

2. Description of the Prior Art Tin-plated steel for food can has been produced by electroplating tin on steel sheet, applying reflow treatment to the thus plated sheet, and then applying a suitable chemical treatment thereto.

More particularly, cold-rolled low-carbon steel sheet basis sheet (to be referred to simplyas basis sheet, hereinafter) is pretreated by cleaning in an alkaline solution, pickling with an inorganic acid solution, and rinsing with cold or hot water. Tin is electroplated on the pretreated basis sheet by using the basis sheet as a cathode in an electroplating bath. In most cases, reflow treatment is applied by resistance heating by feeding electric current therethrough, for improving brightness. Thereafter, the so-called chemical treatment is applied by electrolysis in an aqueous solution containing 6-valent chromium ions.

The conventional tin-plated steel has a shortcoming in that, when being used as food can, it is susceptible to blackening due to sulfurization, which tends to degrade the appearance and the commercial value of the can and canned goods. Such blackening is caused by sulfur-containing foods, such as fish, meat, powdered milk, other protein rich foods, green peas, asparagus, and the like. Accordingly, there is a pressing need for improving the sulfur resistance of tinplate for food can.

Therefore, an object of the present invention is to provide tinplate with a high sulfur resistance, by applying an improved treatment thereto. The inventors noticed that the conventional chemical treatment after the reflow is not effective in providing a desired level of sulfur resistance, and carried out years of studies and experiments to overcome the shortcoming. As a result, the inventors have succeeded in developing a new treatment which overcome the shortcoming.

In addition to the aforesaid foodstuff, fruits and juices (to be referred to as juices, hereinafter) containing organic acids are also canned by using tin-plated steel. Such juices tend to dissolve tin therein. To prevent food poisoning, the concentration of tin in the juices is restricted by law to be below a certain level; for instance, below 150 ppm in Japan, below 250 ppm in England, and below 300 ppm in the U.S.A. On the other hand, the presence of tin in the juice acts to preserve vitamin C therein, to prevent the juice from discoloring, and to enhance the refreshing taste of the juice. Thus, it is generally believed that about 80 ppm of tin in the juice is desirable, although the actual desirable tin concentration varies depending on the kind of juices.

The conventional tinplate tends to excessively dissolve in the juice, so that a suitable coating has been applied to the inside of food can for suppressing the dissolution of tin. Such coating means an additional cost, and it also appears to be detrimental to the taste of the food.

Therefore, another object of the present invention is to provide tin-plated steel whose tin is difficultly dissolvable in juices, so as to avoid food poisoning while ensuring excellent taste of solid or juicy foodstuff to be canned thereby.

The surface treatment according to the present invention also provides a tinplate having excellent physical and chemical properties: namely, low porosity, high brightness (in the case of reflowed tinplate), good lacquer adhesion, appropriate solderability, and excellent stability as determined by thiocyanate test and iron dissolving test.

SUMMARY OF THE INVENTION The present invention provides a surface treatment of tinplate for improving sulfur resistance, which tinplate is particularly suitable for food can, by applying chromate treatment on tin film, immediately after the tin is electroplated on a basis sheet, in an aqueous solution containing 6-valent chromium ions, and then heating at a temperature above C.

With the present invention, the so-called chemical treatment may be further applied to the tinplate after the last mentioned heating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In practising the treatment of the present invention, a basis sheet, i.e., cold-rolled low-carbon steel sheet, is electrolytically cleaned, rinsed with water, pickled with acid, and again rinsed with water in the same manner as in the case of conventional pretreatment for tin electroplating. It is conventional to reflow tin film immediately after the electroplating thereof. On the other hand, according to the present invention, chromate film is formed on the tin layer immediately after the electroplating of the latter, by means of chromate treatment.

The chromate treatment of the present invention may be carried out, for example, under the following conditions.

aqueous solution of sodium Bath composition dichromate at 10 to 50 g/l 0.1 to 3 seconds Cathodic electrolysis by using tinplate as cathode Treating time Polarity The chromate treatment may also be effected by dipping the tinplate in a solution of chromic acid.

After the chromate film is formed, the tinplate is heated at 100C to 400C, preferably at 280C to 350C, for instance by resistance heating with an electric current fed therethrough. Upon completion of the heating, the tinplate is cooled. The heating at below 100C does not produce any noticeable improvement of sulfur resistance of the tinplate, while the heating at a temperature above 400C results in an unacceptably poor formability thereof. The heating temperature of higher than 280C enhances the sulfur resistance, but the heating at above 350C tends to deteriorate the formability of the tinplate.

Instead of using the tinplate as a cathode, it may also be used as an anode or alternately as a cathode and anode in the chromate treatment of the invention. In this case, the sulfur resistance is enhanced, but in the case of reflowed tinplate, the brightness is poor due to the formation of oxide layer thereon prior to the reflow treatment. Accordingly, if high brightness is required, the last mentioned chromate treatment is not suitable.

4 film formed by the anodic treatment in sodium carbonate solution. Accordingly, the deposit of a small amount of chromium oxide before and after the heating seems to produce a surface layer which has ability of The reason why the sulfur resistance can be im- 5 supr ssing any XCe dissolution Of tin. proved by the treatment of the present invention is not Comparative tests were made on various properties fully understood yet. However, the inventors have of tinplates processed by the treatment of the present found out through tests that the treatment of the inveninvention and those of conventional tinplates. The tests tion provides a better sulfur resistance than that which covered sulfur resistance, tin dissolution, porosity (pinis obtained by anodic treatment of tinplate in a solution holes), thiocyanate tests, iron dissolution, oxide film, of sodium carbonate, sodium dichromate. The anodic solderability in terms of capillary rise, lacquer adhetreatment with sodium carbonate will produce an oxide sion, and surface brightness. The result is shown in film, but such an oxide film will not be converted into Table l. The following eight kinds of test specimens, a strongly sulfur resistant film upon heating. it may be i.e., l, 2, 2', 2", 3, 4, 5, and 6, were used in the tests. assumed that the presence of a small amount of chrol. Tin-deposited steel sheets (with tin deposit of 0.50 mium oxide on the electroplated tin film acts to form lb/bb, or pound/base box) were chromate treated by a surface layer which has a high sulfur resistance. cathodic electrolysis in a solution of sodium dichro- With the present invention, it is also possible to apply mate, and then reflow treatment was applied by resisthe so-called chemical treatment to the tinplate after tanee heating at 290C by directly applying electric the aforesaid heating, which tinplate has a kind of tinnt thereto. chromium double layer. The chemical treatment will 2. Tin-deposited steel sheets (with tin deposit of 0.50 further enhance the sulfur resistance and suppress exlb/bb) were chromate treated by cathodic electrolysis cessive dissolution of tin. This chemical treatment i a s l tion of sodium dichromate, and reflow treatmeans the conventional cathodic treatment in an aquement was applied by resistance heating at 290C by dious dichromate solution which is applied immediately tl f din electric current thereto, and then caafter the reflow treatment thodic chemical treatment was applied in a sodium di- The mechanism the improvement of the tin diSSO- chromate solution of g/l concentration at C for lution by the aforesaid chemical treatment is not 1 Second hil k i rr t density at 5 Aldm clearly understood yet. Test result indicates that the 2', Ti -d it d t l sh ts (with tin deposit of chemical treatment providesamuch higher sulfur resis- 30 0 lb/bb) were chromate treated by cathodic electance than that which is obtained by the aforesaid oxide l i i a l i f di dichromate, and

Table 1 Chromate treatment before reflow Thio- ISU in Treat- Sulfur cyanate Porosity iron Specimen treatment Concen- Temper- Current ing resisttest (pinholes/ dissolvtration pH ature density time ance (mg/dm) cm ing test (g/ (se (#g/ m 10 4.5 45 5.0 l SB 4.73 100-150 30 II I! II V I! I! 50 DB 4101 50-100 70 II I! I! It I! 30 3.0 DB 4.25 30-50 4.0 DB 4.57 50 l00 5'0 DB 473 II 6.0 I! II II I! Tin plating Cathodic 4.5 25 DB 4.]7 electrolytic 35 DB 4.09 0.30 Tinplate chromate processed treating 45 DB 4.82 0.5]

by the treatment Reflow DB 4.72 0.48 of the DB 5.46 30-50 invention 45 1.0 DB 4.20 I! I! II 2.5 I! II II II II 5.0 I! I! I! II II I! I! 30 30 5.0 NB 2.3l 0-5 0.58 (2) 45 NB 2.3l 0.51 50 30 NB 0.94 0.51 Tin plating 1 45 NB l.9l 0.29 Cathodic electrolytic 30 0.25 0.2 DB 3.60

or dip chromate 0.5 NB 3.46 5l0 Reflow (2) l.0 NB 3.13 0-5 l 25 NB 185 H Chemical 2 30 0.7 Dipped DB 4.32 20-30 Tinplate Cathode-Anode electrolytic (3) NB 4.98 10-20 chromate reflow I50 Tinplate Anode electrolytic chromate reflow (4) NB 5.38 or more Table 1 Continued Chromate treatment before reflow Thio- ISU in I Treat- Sulfur cyanate Porosity iron Specimen treatment Concen- Temper- Current ing resisttest (pinholes/ dissolvtration pH ature density time ance (mg/dm cm) ing test (g/ e /cm Reference Conventional tinplate (without chemical treatment) (5) HB 6.60 150 0.55

or more Conventional tinplate (with chemical treatment) (6) EB 4.01 10-30 0.55 Conventional .tinplate (with chemical treatment) (6) HB 4.21 -30 0.55

NB: No blackening at all DB: Hardly detectable spotty blackening SB: Slight spotty blackening EB: Slight blackening over entire surface HB: Heavy blackening Table 1A Tin dissolution Solder v Sulfur* Oxide capil- Lacquer Bright- Overall Specimen treatment resistfilm lary adhe ness evaluaance (Condimg ppm (mc/ nse $1011 (71) tion (tion) cm) (mm) SB A 14.6 146 0.96 12 Very 99 Fairly g good DB 13.0 130 0.89 12 97 Good DB 13.3 133 0.85 13 98 Good DB 13.2 132 0.92 10 93 Not good DB A 13.8 138 0.96 12 102 Good DB 14.0 140 1.06 12 101 Good DE 14.1 141 0.99 12 102 Good 1 (1) DB 14.3 143 0.85 10 100 Good Tin plating DB A 13.7 137 0.99 12 98 Good Cathodic electrolytic DB 15.6 156 0.96 13 100 Good Tinplate chromatc DB 12.5 125 0.92 12 99 Good processed treating by th DB 14.6 146 0.92 12 96 Good treatment Reflow of the DB 14.4 144 0.89 11 92 Fairly invention ood DB 14 1 141 0.81 11 93 airly good DB A 15.0 150 0.98 12 100 Good DB 14.4 144 0.99 12 98 Good DB 10.9 109 0.81 12 99 Good DB 14.3 143 0.71 13 98 Good NB A 9.0 90 0.39 14 96 Good (2) NB 8.6 86 0.39 15 99 Good NB 8.8 88 0.35 10 96 Good Tin plating NB 8.4 84 0.32 8 93 Fairly Cathodic good electrolytic DB B 2 5 1 27 13 101 Good clii'bni te NB 1 s 18 0.97 12 99 Good Reflow NB 1.5 15 0.54 99 Good 2 NB 0.8 8 0.64 97 Good Chemical NB 1 O 10 0.48 Fairly 91 Fairly bad bad 2 DB B 2.3 23 1.03 12 Fairly 100 Fairly had good Tinplate Cathode-Anode electrolytic (3) NB B 1.60 16 62 .Bad

chromate reflow Tinplate Anode electrolytic chromate reflow (4) NB 1.73 14 God 52 Bad Reference Conventional tinplate (without chemical treatment) (5) HE A 15.3 153 1.66 14 00d 100 Bad Conventional tinplate (with chemical treatment) (6) EB 14.1 141 1.88 15 God 100 Bad Conventional tinplate (with chemical treatment) (6) HB B 5.5 1.00 12 0d Bad NB: No blackening at all DB: Hardly detectable spotty blackening SB: Slight spotty blackening EB: Slight blackening over entire surface l-lB: Heavy blackening reflowing treatment was applied by resistance heating at 290C by directly feeding electric current thereto, and then cathodic chemical treatment was applied in a sodium dichromate solution with a concentration of 30 g/l at 45C for 1 second while keeping current density at 1 A/dm 2". Tin-deposited steel sheets (with tin deposit of 0.50 lb/bb) were chromate treated by dipping them in a solution of chromic acid (30 g/l) at 80C, and reflow treatment was applied by resistance heating at 290C by directly feeding electric current thereto, and then cathodic chemical treatment was applied in a sodium dichromate solution with a concentration of 30 g/] at 45C for 1 second while keeping current density at 1 A/dm 3. Tin-deposited steel sheets (with tin deposit of 0.50 lb/bb) were chromate treated by anodic-cathodic electrolysis (cathodic treatment at 5 A/dm for 1 second and anodic treatment at 1 A/dm for 0.25 second) in a sodium dichromate solution (30 g/l), and then reflow treatment was applied by resistance heating at 290C by directly feeding electric current thereto.

4. Tin-deposited steel sheets (with tin deposit of 0.50 lb/bb) were chromate treated by anodic electrolysis (1 A/dm ,'l second) in a sodium dichromate solution (30 g/l) at 45C, and then reflow treatment was applied by resistance heating at 290C by directly feeding electric current thereto.

5. Tin-deposited steel sheets (with tin deposit of 0.50 lb/bb) were reflow treated by resistance heating at 290C by directly feeding electric current thereto.

6. Tin-deposited steel sheets (with tin deposit of 0.50 lb/bb) were reflow treated by resistance heating at 290C by directly feeding electric current to them, and then chemical treatment was applied by cathodic electrolysis (5 A/dm 1 second) in a sodium dichromate solution (30 g/l) at 45C.

The process for each test was as follows.

Sulfur resistance test Specimens were dipped in a test solution at 95C for 20 seconds, and blackening due to sulfurization was checked with bare eyes. The test solution contained 8 g/l of potassium polysulfide and 2 g/l of sodium hydroxide. The result was classified into the following 5 grades.

NB: N0 blackening at all DB: Hardly detectable spotting blackening SB: Slight spotty blackening EB: Slight blackening over entire surface HB: Heavy blackening Thiocyanate test This test is to quantitatively determine the amount of iron ions which are dissolved in a corroding solution from exposed steel surface through the pinholes of the tin layer, by converting the iron ions into red thiocyanate for analysis by a colorimeter. The corroding solution contained the following.

ammonium thiocyanate 20 g/l acetic acid 10 g/l hydrogen peroxide solution vol. 70) 10 g/l Rectangular test specimens of 6 cm by 8 cm were prepared and cleaned by organic solvent, such as trichloroethylene and carbon tetrachloride. Cut edges of the test specimens were tightly sealed by wax, and then dipped in the above corroding solution (250 cc) at 20C to 25C for 15 minutes. The color of the solution after the dipping was determined by a colorimeter. The concentration of iron ions, in terms of mg/dm was determined on the basis of a separately prepared calibration curve.

Porosity (pinhole) test Specimens were dipped in a 10% chromic acid solution at C for 5 minutes. After thoroughly washed with water, the specimens were dipped in a distilled water with a pH 4.5 at C for 40 minutes. Thereafter, the number of rust points was counted with bare eyes.

Iron solution test (iron solution value ISV) This test determines the amount of iron dissolved in a corroding solution under the conditions in which tin is slightly more anodic than iron. The corroding solution contained the following.

sulfuric acid 2.18 N 23 cc ammonium rhodanate 40 g1] 25 cc hydrogen peroxide solution 3% 2 cc Total 50 cc The test specimen was placed in a 150 cc glass vessel containing the aforesaid corroding solution, and a threaded plastics lid was so turned as to press the test specimen. An iron-free gasket was used to prevent any leakage.

At the time of colorimeter measurement, 2 cc of hydrogen peroxide solution was added. The aforesaid corroding solution was forced to come in contact with 2,581 mm surface area of the test specimen at 26.7C for 2 hours. Then, the amount of iron dissolved, in terms of ug/cm was determined by the colorimeter.

As the amount of iron thus determined increases (large lSV), the tin-plated steel sheet becomes more susceptible to corrosion.

Tin dissolving test Specimens were vertically dipped in cc of a corroding solution at 25C for a given time period. The difference of the weight of the test specimen before and after the dipping was measured, and the dissolution of tin into solution was calculated in terms of ppm. Condition A degassing of the solution; heat degassing treatment in an argon gas atmosphere corroding solution; citric acid 0.4% (pH 2.8)

NO -N (see Table 5) 100 ppm (0.61 g/l KNO test specimen dimension; 2.5 cm by 10 cm (total surface area of 50 cm edge sealing of test specimen; none dipping time; minutes Condition B degassing of the solution; none (oxygen is dissolved in equilibrium with air) corroding solution; citric acid 0.4% (pH 3) NO N (see Table 5) 20 ppm test specimen dimension; 3.0 cm by 10 cm (total surface area of 60 cm edge sealing of test specimen; sealed with wax dipping time; 60 minutes.

Oxide film The thickness of the oxide film was expressed in terms of the amount of electricity which was necessary for reducing the oxide film on the unit surface area. To this end, the electrolytic reducing process was used with an electrolyte of 0.1 N potassium chloride solution. A test specimen with a given surface area was used as a cathode while carrying it by a holder, and a constant current was fed across the cathode and a carbon rod anode. Whereby, potential-time characteristics curves were obtained, which were used as the basis for determining the thickness of the oxide film.

Solderability test (capillary rise) This test is to check whether solder can permeate into the side seam of a tinplate can. Rectangular test specimens were made, each being 75 mm by 25 mm. The longitudinal direction of the rectangular test specimen coincided with the moving direction of steel during tinplating. Each test specimen was so folded as to form a capillary portion which was surrounded by the tin-plated surface of the specimen. The specimen was dipped in a palm oil at a depth of about mm, which palm oil acted as a flux. Then, the test specimen was dipped into a solder bath at a given temperature by a depth of 30 mm for 1 minute. Upon removal from the solder bath, the specimen was quickly cooled with water.

The solder consisted of 70% of lead and 30% of tin, and the solder bath temperature was 277C. The folded portion was opened after the quick cooling, and the height of capillary rise above the top surface of the solder bath was determined.

The large value of the capillary rise means that the tinplate has a good solderability.

Lacquer adhesion An epoxy phenol varnish was applied to each specimen and baked at 200C for minutes. The varnished surface was scratched in lattice form. The scratched specimen was spherically stretch-formed by a depth of 5 mm by using an Erichsen cupping test machine with a spherical punch (diameter: mm). Trials were made to peel off the varnish layer by a piece of Scotchtape (Trademark of The Minnesota Mining and Machnine Co.). Those specimens which did not allow any peeling were graded as very good.

Brightness The brightness of the specimens of the tinplate as treated by the method of the present invention was measured and expressed in terms of percentage on the basis of the brightness of Reference specimen of conventional tinplates.

As apparent from Table l, the tin-plated steel sheet which is processed by the treatment of the present invention, i.e., the chromate treatment immediately after tin-plating and reflow treatment thereafter, has a much higher sulfur resistance, as compared with that of a conventional tin'plated steel sheet without any chemical treatment. Besides, the number of pinholes, as determined by the hot water test, can be reduced to one half to one third by the treatment of the present invention. The test value of the thiocyanate test can also be reduced to about two thirds by the treatment of the present invention.

On the other hand, the tin dissolution, solderability (capillary rise), lacquer adhesion, and the brightness of the tin-plated steel sheet are not affected by the treatment of the present invention.

If the chemical treatment, e.g., the cathodic electrolytic treatment with a sodium dichromate solution, is applied after the reflow treatment in the method of the present invention, the sulfur resistance can be greatly improved over that of similarly treated conventional tinplate, as can be seen from Table l. The chemical treatment after the reflow is also effective in reducing the number of pinholes and cutting down the thiocyanate test value to less than one half. Furthermore, a remarkable improvement in the tin dissolution can be achieved by the chemical treatment after the reflow, so that the undesirable excessive dissolution of tin can be completely eliminated thereby.

The chemical treatment after the reflow does not affect other properties of the tin-plated steel sheet: namely, it does not deteriorate the iron dissolution, s01- derability in terms of capillary rise, lacquer adhesion, and brightness.

It should be noted that, if the chromate treatment is effected by the anodic-cathodic electrolysis or by anodic electrolysis, the sulfur resistance itself can be improved over that which can be obtained by the cathodic electrolytic chromate treatment. However, the anodiccathodic electrolytic and anodic electrolytic chromate treatments result in an increased thickness of the oxide film, and the succeeding reflow treatment cannot produce the desired brightness of the tin-p1ated steel sheet.

Immediately after the tin electroplating, a chromate coating is formed on the electroplated tin without reflowing the latter. Thereafter, the tinplates were heated at C to 300C by electric resistance heating by directly feeding electric current thereto. The relation between the heating temperature and the sulfur resistance of the tinplates thus treated was measured. The test included the plates which were not heated at all after the chromate treatment and plates with electroplated tin alone. The result is shown in Table 2.

Table 2 Duration** Sulfur resistance of specimens heat at*** Test* of sulfur specimen resistance without test (sec) heating 50C C C C 200C 240C 280C 320C 20 DB DB NB NB NB NB NB NB NB A 40 SB SB DB DB DB DB DB NB NB 20 B HB Test specimen A:

lmmediately after tin electroplating 1.50 lb/hb). cathodic electrolytic chromate treatment was applied in a solution of sodium dichromate.

Test spccimcnt B: Plates with tin electroplating alone (0.50 lb/bb). "Sulfur resistance test was the same as that of Table 1.

*(irading notations of the sulfur resistance are the same as those of Table l.

As apparent from Table 2, in the case of tinplates which are chromate treated immediately after the tin electroplating, the heating at a temperature above 100C is effective in improving the sulfur resistance even when the heating temperature is below the melting point of tin.

The reason for the improvement of the sulfur resistance is not clearly understood yet. Judging from the fact that the heating of tinplate with tin coating alone did not produce any improvement of the sulfur resistance, the oxides of chromium deposited on the tin layer by the chromium treatment produce a highly sulfur-resistant layer upon heating.

The invention will now be described in further detail, by referring to examples.

EXAMPLE 1 EXAMPLE 2 Basis sheets of cold-rolled low-carbon steel was pre- 10 treated by electrolytic cleaning, rinsing with water,

pickling with acid, and rinsing with water, and they were tin-plated (5.6 g/m and rinsed again with water. Chromate treatment was applied to the thus tin-plated steel by cathodic electrolysis in a solution containing 15 g/l of sodium dichromate at C with a current density of 0.5 A/dm for 0.2 second. Then, reflow treatment was applied by electric resistance heating at 290C by directly feeding electric current thereto, which was followed by chemical treatment of cathodic electrolysis in a sodium dichromate solution of 30 g/l at 45C with a current density of l A/dm for 1 second. Food cans of inside volume 152.5 cc, diameter 65.4 mm, and height 52.7 mm were made from the tinplates thus prepared, and reference food cans of the same size Table 4 Tests in 2 w cysteine chloride solution Tin dissolution (ppm) Can inside corrosion by sulfur After storing After storing Treatment Immediately at 38% for immediately at 38C for after heat after heat sterilization l 2 3 sterilization l 2 3 week weeks weeks week weeks weeks The Slightly invention 9 61 Normal Normal Nonnal colored Conventional 21 103 128 155 Considerable blackening was noticed density of 5 A/dm for 0.5 second. Then, reflow treatment was applied to them by electric resistance heating at 290C by directly feeding electric current thereto, which was followed by a chemical treatment of cathodic electrolysis in a sodium dichromate solution of were made from conventional tinplates which were processed by conventional chemical treatment.

Accelerated tin dissolution tests were made on those cans by using cysteine chloride solution and citric acid solution. The result is shown in Tables 4 and 5.

Table 3 Sulfur resistance, as determined by Treatment Cysteine Sodium Potassium chloride thiosulfate polysulfide process process process Tin-plating chromate by The cathodic electrolysis Normal Normal Normal invention reflow chemical treatment Conventional tinplate (0.50 lb/bb), Considerable Considerable Considerable with chemical treatment blackening noticed blackening noticed blackening noticed Table 5 Tests in citric acid solution Tin dissolution (ppm) immediately After keeping Treatment Corroding solution after heat at 38C for sterilization 24 hours Citric acid pH 3.0 42 79 The do. +NO N 5 76 Table Continued Tests in citric acid solution Tin dissolution (ppm) *NO N stands for the amount of nitrogen which was added in the form of potassium nitrite.

What is claimed is:

1. Surface treatment of tin-plated steel for a food can and the like for improving sulfur resistance and producing a bright finish, comprising plating tin on the surface of steel, applying cathodic electrolytic chromate treatment to the tin thus plated by using an aqueous acid solution containing 6-valent chromium ions and whose pH value is 3 to 6 prior to reflowing and treating the chromate treated tin-plated steel at a temperature from 280 to 350C.

2. Surface treatment for tin-plated steel for a food can and the like for improving sulfur resistance and producing a bright finish, comprising plating tin on the surface of steel, sujecting the thus plated tin to a dip chromate treatment with chromic acid prior to reflowafter the heating step. 

1. Surface treatment of tin-plated steel for a food can and the like for improving sulfur resistance and producing a bright finish, comprising plating tin on the surface of steel, applying cathodic electrolytic chromate treatment to the tin thus plated by using an aqueous acid solution containing 6-valent chromium ions and whose pH value is 3 to 6 prior to reflowing and treating the chromate treated tin-plated steel at a temperature from 280* to 350*C.
 2. Surface treatment for tin-plated steel for a food can and the like for improving sulfur resistance and producing a bright finish, comprising plating tin on the surface of steel, sujecting the thus plated tin to a dip chromate treatment with chromic acid prior to reflowing and heating the chromate treated tin-plated steel at a temperature from 280* to 350*C.
 3. Surface treatment of tin-plated steel according to claim 1, wherein said heating of the chromate treated tin-plated steel is effected at temperatures from the melting point of tin to 350*C.
 4. SURFACE TREATMENT OF TIN-PLATED STEEL ACCORDING TO CLAIM 1 FURTHER COMPRISING APPLYING AN AQUEOUS ACID SOLUTION CONTAINING 6-VALENT CHRONIUM IONS AFTER THE HEATING STEP.
 5. Surface treatment of tin-plated steel according to claim 2 further comprising applying an aqueous acid solution containing 6-valent chromium ions to the tin after the heating step. 